1. Field of the Invention
The present invention relates to a dot matrix type liquid crystal display apparatus, and more particularly, to a liquid crystal display apparatus driven by a voltage averaging method.
2. Description of the Prior Art
In a liquid crystal display apparatus using a so-called simple matrix type liquid crystal cell, conventionally, a scanning circuit and a driver circuit (these circuits are generally named as row and column circuits for driving row and column electrodes which are referred to as scanning and signal electrodes in this specification) are connected to the liquid crystal cell, and a selecting voltage and a non-selecting voltage are supplied from a power supplying circuit to drive the liquid crystal cell by a voltage averaging method. With this arrangement, however, an undesired display called a ghost appears as described in Japanese Laid-open Patent Application No. H2-245726. The ghost is a weak shadow-like display appearing on an extension of display pixels (selected pixels) when, for example, a bar chart or a frame is displayed.
To overcome such a problem of the ghost, a method in which the bias voltage is changed and a method in which a capacitor is provided have been used for a long time. For example, the above-mentioned Japanese Laid-open Patent Application proposes to increase a scanning side non-selected bias voltage. In another known method, variation in each bias voltage is absorbed by connecting a capacitor between a reference bias voltage line and another bias voltage line. However, since these methods do not always realize sufficient voltage compensation, although no problems arise in the case of a liquid crystal display apparatuses having a small number of display pixels, it is impossible to restrain the remarkable appearance of ghosts in liquid crystal display apparatuses having a large number of display pixels. Moreover, even in the case of the liquid crystal display apparatuses having a small number of display pixels, ghosts remarkably appear when the ambient temperature varies.
Hereinafter, one of the factors which are presumed to be reason why the ghost appears more remarkably when the number of display pixels is large will be described. Generally, the relationship between a liquid crystal applied voltage and a light transmittance is represented by the characteristic shown in FIG. 16. The axis of the abscissa of FIG. 16 represents a voltage applied to the liquid crystal cell, and the axis of the ordinate represents a light transmittance T. Numeral 61 represents a light transmittance curve of an ON pixel, and 62 is a light transmittance curve of an OFF pixel. Vop represents a normal working voltage, which represents a peak value of the selecting voltage for the ON pixel and a peak value of the non-selecting voltage for the OFF pixel. When the number of scanning electrodes of a liquid crystal display apparatus is small and a time-division number N is small, since the ON and OFF curves are sufficiently away from each other as shown in FIG. 17, even if the ON and OFF curves change to 61a and 62a, 61b and 62b and 61c and 62c due to a variation in voltage, no problems arise since the working voltage Vop intersects constant transmittance portions of the curves. However, when the number of scanning electrodes of a liquid crystal display apparatus increases and the time-division number N also increases, the distance between the ON and OFF curves decreases as shown in FIG. 18. Since the working voltage intersects inclining portions of the curves 61a, 61b, 61c, 62a, 62b and 62c for this reason, when the voltage varies, a point T.sub.ON at which the pixels are ON and a point T.sub.OFF at which the pixels are OFF vary to points .DELTA.T.sub.ON and .DELTA.T.sub.OFF as shown in FIG. 18, so that the contrast (a ratio of T.sub.ON to T.sub.OFF) changes. On the other hand, in a case where the selecting voltage is applied to specific continuous pixels such as where a bar chart is displayed in a part of an image plane, voltage variations are prone to be generated in the non-selecting voltage which counteracts the selecting voltage at pixels located in an extension of the display. At this time, ghosts appear under a condition where the above-mentioned contrast variation is readily caused.